I decided to start by simply copying the finalized list of repos that I had developed and posted in that earlier note. This replaced my existing /etc/apt/sources.list. I wasn't sure if I needed to reboot, but having done all those updates and such, I decided it couldn't hurt. On reboot, I went into System > Administration > Software Sources to see what we had. There were some items on the list that had to do with individual programs. I found another such list that was even longer. I wasn't sure why I needed repositories for individual programs; I was under the impression that repositories were places where you could get a variety of programs. So rather than add to the list (and increase the number of repositories that might report problems later on, as I had experienced previously), I pared down the list. My final list, preserved in a backup copy of /etc/apt/sources.list, was as follows:

I clicked Close. It gave me a chance to Reload, which I took. It said this:

Could not download all repository indexes
The repository may no longer be available or could not be contacted because of network problems ...

I closed out of that and typed "sudo gedit /etc/apt/sources.list." Sources.list still had some lines for repositories that I had deleted, so I deleted those surplus lines. It also had commands I should run in connection with the rest, so I copied those lines into a separate file in gedit, saved it as RunForRepositories, made it executable ("sudo chmod +x RunForRepositories"), saved a copy for future use, and then ran it ("sh RunForRepositories). I went back into Software Sources, changed one thing, closed, reloaded, and got the "Could not download all repository indexes" error again. At this point, I rediscovered that you could add repositories via System > Administration > Synaptic Package Manager. In Synaptic, I went to Settings > Repositories. But this turned out to be the same thing as the Software Sources tool. In Software Sources > Other Software, I unclicked and then reclicked an item to trigger the Close > Reload option. Once again, I got the "Could not download all repository indexes" error. The first item on the list was the Medibuntu repository, with this message: "The following signatures could not be verified because the public key is not available." Following advice, I closed out of that, went into Synaptic, searched for medibuntu-keyring, and applied that. In Synaptic, I went again into Settings > Repositories and retriggered the Reload. But it didn't work -- no Reload option -- so I did it via System > Administration > Software Sources. This time the Medibuntu error was gone, and the first error in the list had to do with the Ubuntu CD. I decided I didn't want the Ubuntu CD to be a source, so in Software Sources I unchecked the CD on the Ubuntu Software tab, and then retriggered the Reload option again. No more errors.

For some reason, /etc/apt/sources.list did not show some of these changes. Maybe I had it open in a separate window while I ws going through my paces. So I edited it manually to reflect my current preference and resaved a copy on a separate partition. Now that this was done, I ran System > Administration > Update Manager and installed the new updates they had for me there. I clicked Check to repeat that step. It said my system was up-to-date. I concluded that the repository step was done.